The invention relates to rotatable data storage devices and more particularly to magnetic disk files having rotatable magnetic disks across which transducers are moved for the purpose of reading from and writing on the disks and for the purpose of determining the particular magnetic tracks on the disks on which the transducers are located.
It has been previously proposed in said John R. Taylor U.S. Pat. No. 3,936,876 that a velocity circuit be actuated by a position error signal (which has a zero crossing for each track crossing) derived from a servo transducer moving across the tracks of a magnetic disk, with the velocity circuit cooperating with other circuitry so as to bring the transducer accurately to a target track on the disk in a track crossing access operation. The John R. Taylor circuits include a filter which has a relatively low roll off so that undue noise is not transmitted to the velocity circuit which has a differentiator therein that would otherwise provide erroneous velocity signals.
The John R. Taylor circuitry has proved to be very satisfactory for transducer velocities quite acceptable to the present time. The Taylor circuitry was designed for a maximum transducer velocity of 1320 tracks per second, and it was also satisfactory for a subsequent maximum transducer velocity of 2381 tracks per second. However, when the transducer speed was increased to 7143 tracks per second, it was found that changes were necessary.
At the latest maximum transducer speed of 7143 tracks per second, it has been found that the roll off of the Taylor filter should be increased so that a non-degraded position error signal be transmitted to the velocity circuit and so that the velocity circuit may accurately indicate the transducer velocities at these high velocities during a track access. As the transducer slows, however, in approaching its target track, the velocity circuit has been found to provide erroneous velocity indications, this being due to considerable noise transmitted through the relatively high roll off filter and differentiated by the differentiator. For high speeds of the transducer across the disk, the signal to noise ratio of the output of the differentiator is high; however when the transducer slows in reaching its target track, this signal to noise ratio is low since the gain of the differentiator is proportional to frequency, causing erroneous velocity indications by the velocity circuit and particularly by its differentiator. The noise is considered to be due to defects in the disk, resonances in the circuitry and in the mechanical supporting structure for the transducer and variations in spacing between the head and the disk as the disk rotates (the transducer flies at a few microinches over the disk during operation).